Even when they have a register/computer calculating the change, many times they screw up that transaction, too. Give a clerk a $10 bill and a $1 bill for a $5.95 order, and many will, after looking at the two bills for a bit, hand you back the $1 bill, key in the $10 bill like they are smarter than you, and give you the additional $4.25 in change. Some who manage to actually key in $11 are surprised at the outcome.
^^^ $4.05
I remember years ago giving the owner of a small Chinese restaurant a $20 dollar bill and 13 cents for a 24.88 tab. He looked at me kind of strange but just keyed it in, but he went ahhhh when he got the result. I figured he was too busing scrambling around running the place to figure it out in his head. My wife told me she hated when people did what I did.